WASHINGTON — The universe of potential changes to federal gun laws seemed to shrink Wednesday during an occasionally fraught Senate hearing on gun violence as lawmakers and proponents of more gun rules tussled with gun rights advocates over the availability of some types of weapons and ammunition. In the end, chances for a ban on assault weapons dimmed, and compromise seemed elusive.

The hearing, the first held by the Senate Judiciary Committee since the mass shooting last month in a Newtown, Conn., school, began on a poignant note as former representative Gabrielle Giffords, who was critically injured in a 2011 shooting, addressed the committee slowly and with passion, essentially begging them to come up with legislation to address gun violence.

‘‘Too many children are dying,’’ she said to a packed, hushed hearing room. ‘‘Too many children.’’

After Giffords’ brief testimony, the four-hour hearing quickly devolved into a litany of competing statistics and chilling anecdotes, laying bare the deep national divide between those who believe gun availability contributes to the nation’s most violent crimes and those who think it helps prevent them.

Wayne LaPierre, the chief executive of the National Rifle Association, spoke ruefully of the many years he has spent trotting to Capitol Hill to testify about gun violence, and grew irritated under the questions of friend (Senator Ted Cruz of Texas, a vocal supporter of gun rights) and foe (Senator Richard J. Durbin of Illinois, where an honor student in Chicago was fatally shot hours before the hearing).

LaPierre said he did not support the measure that appeared to be gaining the most support among both parties — enhanced background checks for gun buyers — raising the prospect that perhaps even modest changes to gun laws would be hard to accomplish.

‘‘Universal background check, which sounds, whatever,’’ he said, ‘‘ends up being a universal federal nightmare imposed upon law-abiding people all over this country.’’

LaPierre’s strong defense of existing guns laws, which he argued were poorly enforced, and his occasional pique were a contrast with Giffords’s husband, Mark Kelly, a gun-owning former Navy captain and a retired astronaut, who quietly pulled at the arguments against stronger background checks, which he and Giffords seek.

‘‘I’ve been shot at dozens of times,’’ Kelly said. ‘‘I would suspect that not many members of this panel, or even in this room, for that matter, have been in any kind of a firefight. It is — it is chaos. I think there are really some very effective things we can do. And one is, senator, the background check. Let’s make it difficult for the criminals, the terrorists, and the mentally ill to get a gun.’’

The greatest area of disagreement centered around the availability of so-called assault weapons, which some Democratic senators seek to ban, and restrictions of large-size magazines, which several Republican senators and their witnesses argued would endanger potential victims of crime and infringe on the rights of law-abiding Americans.

Giffords, who made her way through the hearing room slowly, passing by several senators to bid them hello and give them a kiss, sat next to her husband and slowly began her remarks.

“This is an important conversation for our children, for our communities,’’ Giffords said. ‘‘For Democrats and Republicans. Speaking is difficult, but I need to say something important. Violence is a big problem,’’ she continued.

‘‘We must do something. It will be hard. But the time is now,’’ she said, emphasizing the last word. ‘‘You must act. Be bold. Be courageous. Americans are counting on you.’’

With that, Giffords made her way quietly out of the room.

‘We must do something. It will be hard. But the time is now. You must act. Be bold. Be courageous. Americans are counting on you.’

Senator Patrick J. Leahy, Democrat of Vermont, chairman of the committee, began by noting that ‘‘the Second Amendment is secure and will remain secure and protected.’’ He added: ‘‘Americans have the right to self-defense and to have guns in their homes to protect their families.’’

Leahy, who has a record of supporting measures like an assault weapons ban but also of defending the rights of gun owners, said: ‘‘No one can or will take those rights or our guns away. But lives are at risk when responsible people fail to stand up for laws that will keep guns out of the hands of those who will use them to commit mass murder. I ask that we focus our discussion on additional statutory measures to better protect our children and all Americans.’’

Leahy promoted his own bill, which would give law enforcement officials more tools to investigate so-called straw purchasing of guns, in which people buy firearms for others. But he did not push for a ban on assault weapons, and except for Senator Dianne Feinstein of California, who has her own bill in which a ban renewal is central, most people’s comments focused on background checks and mental health provisions to prevent the wrong people from obtaining guns.

Senator Charles E. Grassley, Republican of Iowa, set the tone for gun-rights advocates, by noting that while the tragedy of Newtown has shocked and rattled the nation, the events should not ‘‘be used to put forward every gun-control measure that has been floating around for years.’’

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